This invention relates to a method of alloying aluminum into lead, and in particular to a method of alloying aluminum powder into lead to form lead-calcium-aluminum alloys.
Lead-calcium-aluminum alloys are used to make battery grids. The presence of aluminum in the alloy helps to protect the calcium content of the alloy during formation of the grids. However, alloying aluminum into lead has long been a problem. In most alloying operations at least some of the alloying material is lost to the formation of slag or otherwise. The difficulty of, and low recoveries from, the direct alloying of aluminum powder or molten aluminum into lead is well known. "Recovery" as used herein refers to the percent of the alloying material that is successfully alloyed. Furthermore, the alloying of aluminum into lead had previously resulted in unpredictable recoveries. That is the standard deviation for the aluminum recoveries in present alloying techniques is quite high, and thus it has been difficult to achieve a preselected aluminum content in one alloying step with any certainty. One or more supplemental alloying steps were often required to bring the aluminum content within the preselected range. These supplemental alloying steps greatly increase the cost of making such alloys because they tie up the refining equipment and personnel, and disrupt scheduling in the plant.
Presently, most lead-calcium-aluminum alloys are made by one of two methods: (1) dropping brickettes of a calcium aluminum mixture into the molten lead and stirring, (2) dropping chunks of calcium and pouring molten aluminum into molten lead and stirring. These alloying reactions are strongly exothermic, and generate so much heat that the lead actually burns, forming lead oxide fumes, which can be a health hazard. Moreover, the calcium may also fume, forming caustic fumes which can be an eye and skin irritant. For these reasons alloying is sometimes conducted under a ventilated cover, but this is cumbersome.
Furthermore, the recoveries of calcium and aluminum with these methods of alloying are relatively low and unpredictable. For example, the inventors have found that use of calcium aluminum brickettes on a commercial scale typically results in recoveries of calcium of 89%.+-.5.27% and recoveries of aluminum of 71.7%.+-.17.66%. Thus within the first standard deviation the actual calcium content for any given alloy made by this method can vay by over 10% and the actual aluminum content for suqh an alloy can vary by over 34%. With such large variations it is difficult to reach a preselected content in just one alloying step without the need for adjustment. Similarly, the inventors have found that use of lump calcium and molten aluminum on a commercial scale typically results in recoveries of calcium of 84.7%.+-.7.36% and recoveries of aluminum of 55.26%.+-.10.69%. Thus within the first standard deviation the actual calcium content for any given alloy made by this process can vary by over 14% and the actual aluminum content of such an alloy can vary by over 21%. Again, with such large variations it is difficult to reach a preselected content in just one alloying step without the need for adjustment.
Because of the relatively high alloying losses, and the unpredictability of the alloying reactions evidenced by the relatively high standard deviations associated with the recoveries (particularly aluminum) the cost of obtaining a given alloy composition has been quite high. After a first attempt at reaching a specified alloy composition, a sample must be drawn and analyzed. If the content is not within the specifications one or more supplemental alloying operations are required to bring the content within specification. The inventors have found that supplemental alloying steps are needed more than 50% of the time. This is costly not just in terms of the additional alloying material needed, but these supplemental alloying operations tie up refinery equipment and personnel, and interfere with scheduling.
There have been suggestions in the art to add calcium to molten metals, such as to steels for purification, by injecting specially made calcium granules into the molten metal under the surface. It has even been suggested, but to the inventors' knowledge never previously proven, that calcium could be alloyed into lead by such a method. However, such a method of alloying calcium into lead makes no provision for the addition of aluminum to protect the calcium content of the alloy, and thus was of little or no use. Furthermore, there was no indication whether the uncertainties of the recoveries would be in any way improved. Calcium granules containing alloyed aluminum became available, but the maximum aluminum content of such granules was very low, about 12%. Thus, obtaining a sufficient aluminum content to protect the calcium would have required a large amount of such granules, would have wasted a great deal of calcium, and would have been prohibitively expensive. Given the well-known problems of adding aluminum, even molten aluminum, to lead, there appeared to be no satisfactory way of adding aluminum to lead with this method.